In a conventional form of fuel injection system, fuel is injected into an engine cylinder through a fuel injector mounted on the cylinder head. One known form of a fuel injector is a needle-lift injector which responds to a pulse or charge of pressurized fuel by opening and permitting the pressurized fuel to be injected into the cylinder through the duration of the pulse. In a compression ignition (diesel) engine, the timing of the pulse and its duration must be carefully controlled to correlate injection timing and fuel quantity with the actual and commanded speeds of the engine at a given set of engine operating conditions.
One known method for controlling injection timing and fuel quantity or metering is to spill back to drain some of the fuel produced through the power stroke of each cycle of a high-pressure fuel injection pump. More specifically, by using only a preselected interval within the power or work stroke of the pump, a pressure pulse having appropriate timing and duration in relation to engine crankshaft position and actual and commanded engine speeds can be produced. Although this principle of spilling fuel back to drain may intuitively seem wasteful, it has proven to be highly effective, especially for fuel injection systems with piston-type pumps that are synchronized with the engine crankshaft. The power stroke of a high pressure piston pump is generally long enough to permit the advance or retard of injection and to accommodate the desired range of fuel metering. The remaining portion of the power stroke is not used to provide fuel to the cylinders, and the pressurized fuel produced by the pump during this portion can be relieved or spilled back to drain through a spill valve connected to the fuel pump. The spill valve is normally connected in a spill path between the pump chamber and drain.
The actuation of the spill valve determines which preselected portion of the power stroke will be used for fuel injection. During the preselected portion of the power stroke that is to be used for fuel injection, the spill valve is closed and the spill path communicating the pump chamber and drain is interrupted. However, during the remaining portion of the power stroke that is not used for fuel injection, the spill valve is opened and the spill valve communicates the pump chamber to drain.
It is important that the actuation of the spill valve between its closed and open conditions be relatively fast so that the timing of injection and metering of fuel can be closely controlled. A conventional method of actuating a spill valve between its closed and open conditions is to appropriately energize or deenergize a solenoid relay in the spill valve that controls the opening and closing of a spill path through the valve. There are, however, certain limitations associated with a conventional solenoid relay in this application. Specifically, a conventional solenoid relay has a response time limited by the time required for the buildup of a magnetic field and the dissipation of residual magnetism. In addition, inertial effects can also delay the response time of a conventional solenoid relay.
An example of a prior art device employing a solenoid relay is the distributor-type fuel injection system disclosed in Twaddell et al, U.S. Pat. No. 3,880,130, issued Apr. 29, 1975. The Twaddell et al device employs two complementary solenoid relays that cooperate to open and close a spill line connected to the chamber of a distributor pump. Each solenoid relay functions independently of the other solenoid relay and has a response time determined by its separate characteristics. Also of interest are the teachings contained in the patents to Eheim, U.S. Pat. No. 3,841,286; Watson et al, U.S. Pat. No. 3,859,972; and Murtin et al U.S. Pat. No. 3,851,635.
Where response time limitations are present, they are multiplied in effect when a single spill valve unit is used to control the fuel injection and metering of a plurality of cylinders. More particularly, in a distributor-type injection system, where a pump is connected to a parallel injector network and the spill valve is connected to the pump upstream of the injector network, the spill valve must go through a complete spill cycle for each cylinder fed by the injector network. Stated otherwise, for a four-cylinder, four-cycle engine, the spill valve must complete two spill cycles through each revolution of the engine crankshaft. At relatively high engine speeds, e.g. 6,000 RPM, the spill valve must have relatively fast actuation and deactuation times to maintain precision control over injection time and fuel metering for each cylinder. This requirement poses a severe timing requirement for spill valves using conventional actuation devices.
An objective, therefore, of the present invention is to provide a spill valve with a relatively fast response time that can be used as a single point control in a fuel injection system to precisely control the injection and metering of fuel through a high range of engine speeds.